This invention relates to methods, apparatus and systems utilizing and benefitting from the energy of pulses, oscillations and impacts on an exterior surface of a solid work product to rearrange the interior product structure, typically by accompanying plastic deformation. Thus, ultrasonic energy is employed for treatment of metallic and plastic bodies with and without welds. Typically bodies of ferromagnetic metal structures are treated on exterior surfaces. More particularly this invention relates to reduction, elimination, redistribution, relaxation of tensile stresses and defective structures such as voids and grain structures weakening the internal body structure including residual welding stresses. Defects tending to cause structural fatigue and failures in metallic structures and welded products are thus treated by the impact of ultrasonic energy applied to the work product external surfaces. Work products, typically product structures and welded products, are nondestructively impacted at exterior surfaces in the vicinity of welding joints when present, e.g. at welding toes, and/or to non-welded surfaces to thereby restructure the work product internal stresses to impart longer life and greater weight bearing strength.
In the metal forming and welding arts, the initial manufacturing process, the after-manufacture treatment of the product, the encountering of and the magnitude of loads in use and the aging process lead to deterioration of load bearing strength in the product structure, whether unseen without destructive analysis or evidenced by catastrophic failure, such as by appearance of fractures or cracks.
Conventional welded products are made by employment of various welding art technological operation steps before and after the actual welding step in an attempt to improve the working life of the products. Some of these technological operation steps are categorized as: (a) pre-welding preparation of exposed surfaces at welding sites by abrasive or chemical cleaning, (b) post-welding processing of welded seams by cleaning flux and slag and by surface shaping to remove visible sharp projections and contours that identify concentrated stress areas, (c) surface treatment of the welded structure with corrosion resisting coatings, (d) thermal tempering for relaxation of residual stresses and for internally restructuring the metal grain in a manner reducing the influence of stress concentrations, and (e) demagnetizing treatment to protect welding arcs from magnetic interferences during multi-pass welding operations.
There are interactions of the various independent steps typically occurring at various times on metal products, particularly in view of various intricate work product shapes and loading patterns, and the difficulties in detecting defective subsurface base material patterns, such as grain structure and residual stresses in the product that affect fatigue, life and strength, particularly in the presence of stress concentration zones and highly loaded working zones. Thus, efforts in combatting long term fatigue initiated both during initial manufacture and during useful life with various technical operations heretofore available in the prior art have been substantially limited in their effectiveness and/or are unpredictable, thus producing compromised product quality inconsistent with expected and desired performance.
Known vibration and pulsed methods of stress relief include inducing low frequency mechanical vibrations into products such as welded structures to reduce residual stresses, and employing pulsed magnetic fields to relieve stress in ferromagnetic cutting tools.
At this stage of the prior art, a number and variety of interacting technical operations in a series of processing steps in initial production are required to fabricate proper welded metal products with greater load bearing capacity and lower internal residual stresses for longer expected life and higher quality. Simplification and lower cost of the production process as well as improved performance is thus highly desirable.
Welded metal product or structure manufacturing and repair practices require the addition of and/or removal of materials which therefore are consumed in the manufacturing process. For example, overlay welding and beading operations for strengthening weld seams require more initial product metal and require additional technical operations such as mechanical grinding, removal of fluxes and residues, thermal tempering and cosmetic shaping. It has not been feasible to obtain optimum appearance, strength and life in welded products without such steps. On the other hand, such steps increase costs of production and result in more complex fabrication process.
It is conventional to retire and replace aging metal structures such as steel bridgework and load bearing products subject to aging, which encounter stress fatigue corrosion, undesirable internal stress patterns, and the like, causing the presence of either unseen internal damage or observable surface defects. It is therefore desirable to provide improved maintenance and repair technology to extend the useful work product life by restoring or improving initial load bearing strength and reducing residual stresses in maintenance procedures so that current structures may be kept in operation.
In the welding structure arts conventionally in practice, practical technology has not been available which is well adapted for in-use non-destructive and non-deforming repairs to restructure and restore welded products that have become structurally unsound from aging, that have reduced loading capacity because of fatigue and residual stresses, or which have catastrophically failed by cracking, or the like.
For example, the prior art ability to repair visible catastrophic failures of structure, evidenced by cracks or fractures, in most part is limited to the addition of supporting braces, crutches, and other types of overlying structure to bypass damaged zones. Such techniques are not suitable for many metallic structure installations where there is either no accessible place to rework the welded products in-situ, where restrictions in space are imposed or where appearance of such bypassing structure is intolerable such as in bridgework and building structural support infrastructure.
One zone subject to residual stresses which may cause early failure is the junction zone between basic metal material and weld seams that may contain residual grain or stress patterns formed in the welding process. There are prior art techniques for annealing to redistribute and relax the stress patterns. However in general this is not a scientific method but an art dependent upon skills and experience of a few artisans, such as blacksmiths, where access to the work product is available. Such artistic methods have been applied for example in tempering knives or swords. One significant reason that such methods have not been replaced by scientific technology is that the nature of internal structure is difficult to ascertain and stress concentrations are of a diverse nature that defy analysis.
Thus, a serious deficiency with the manufacture and repair of structural and load bearing products is the lack of non-destructive detectors and corresponding automated systems that can both sense the nature of internal defects and correct them in diverse kinds of internal work product structure by restoring structural integrity to produce longer life following original manufacture procedures or renewed life imparted in maintenance procedures that overcome fatigue and internal stress patterns reducing product performance.
Accordingly, a specific objective of this invention is the introduction of novel procedures for sensing the nature of interior body grain and stress patterns, which is particularly important when involving metallic and ferromagnetic product lines either with or without welding seams.
Also detection of internal product structural conditions provides a frontier for novel automation procedures for radically improving the initial manufacturing phase of metallic or plastic bodies which are subjected to mechanical and thermal stresses in use.
Accordingly it is an objective of the present invention to correct such foregoing disadvantages of the prior art and to introduce production, maintenance and repair technology which can produce work products having idealized internal structure with improved load and wear capacity by elimination of residual stresses, voids and inferior grain structures that reduce product life.
Examples of typical prior art technology related to this invention or teaching some of the elementary underlying methodology now are briefly set forth, which in the present invention are interactively combined to produce novel combinations of technologies as a whole.
Overlay technology exists, such as filler welds and overlay welds, wherein strengthening elements are superimposed over critical zones to bypass fatigued, fractured or other deficient welded product structure. The overlay may be superimposed directly upon weld seams in some cases. Typical examples of this technology are U.S. Pat. No. 2,537,533, G. E. Ingalls, Jan. 9, 1951; U.S. Pat. No. RE 16,599 R. Mattice, Apr. 19, 1927; U.S. Pat. No. 1,703,111, S. J. Kniatt, Feb. 26, 1929; and U.S. Pat. No. 1,770,932, A. G. Leake, Jul. 22, 1930. Such overlay structure in U.S. Pat. No. 4,049,186 R. E. Hanneman, et al., Sep. 20, 1977 and U.S. Pat. No. 4,624,402, D. Pitcairn, et al., Nov. 25, 1986 in particular disclose overlay welds for the purpose of preventing stress corrosion failures in the welded body.
Peening by means of pellets, hammers, stress waves and ultrasonic impact is known to surface treat and deform the welded body surface structure for contouring weld sites to induce plastic deformation producing beneficial effects and heating of the metal for thermal tempering effects. Typical art of this nature includes U.S. Pat. No. 5,654,992, K. Uraki, et al., Aug. 5, 1997 and U.S. Pat. No. 3,961,739, B. P. Leftheris, Jun. 8, 1976. These disclosures recognize that mechanical pressure and stress waves applied to the external surface of a body creates thermal energy and a momentary state of plasticity in the workpiece.
In U.S. Pat. No. 4,330,699, N. M. Farrow, May 18, 1982, a non-contact laser welder is accompanied by a second amplitude modulated laser for generating acoustic waves in the melt to improve interdiffusion and homogeneity of the weld joint.
I have authored or co-authored several publications relating to ultrasonic impact treatment of welded joints and the relationship to fatigue resistance, typically as reported in the following International Institute of Welding IIW Documents:
Publication XIII-1617-96 for example discloses that the fatigue strength of as-welded joints was increased by changes in mechanical properties of material in surface layers induced by ultrasonic impact treatment (UIT). Thus, the material at the weld toe is compressed and deformed by manual indentation using an ultrasonic probe to form indented groove structure smooth and free from irregularities. This technique depends upon the training and skills of an operator manually wielding an ultrasonic probe to form the grooves, and requires reshaping of the weld site.
The comparison of peening with (UIT) is discussed in Document XIII-1668-97, which sets forth the advantages of ultrasonic impact treatment technology over peening, and the practicability of UIT technology to compress and indent the welded body structure in the vicinity of the weld seam.
The use of ultrasonic hand tools for achieving foregoing compression indentations is set forth in Document XIII-1609-95.
These techniques have demonstrated significant increases in fatigue limits of welded structures. However, this prior art technology requires physical distortion of the welded product or structure, and demands skilled labor to make decisions on the nature of indentations in the presence of different physical shapes of welded bodies and different loading requirements at the weld sites. Thus, it is neither practical nor economically feasible to apply the techniques universally or by automation to welded products of various sorts. Furthermore there can be no consistency from one product to another to assure constant quality performance expectations. Nor can techniques provided for initial welding production cycles only, be used for later maintenance of welded products or for repairs of cracks and other catastrophic failures.
The present invention has the objectives of curing deficiencies in the aforementioned type of prior art, and offering significant advantages in simplifying processing steps while guaranteeing higher quality products and improving useful life span and higher loading capacities of welded products at various stages of life, throughout the initial production of the product and even after catastrophic failures appear, such as visually observable cracks.
A significant objective of the invention is to coordinate and combine non-destructive ultrasonic impact treatment of work product bodies without deforming their shape in a procedure applicable to manufacturing, maintenance and repair processes, typically to relax internal stresses, reverse fatigue effects, improve corrosion fatigue strength and durability of load bearing surfaces and joints, and to create relaxed more ideally distributed internal body stress patterns.
It is a specific objective of the invention to introduce improved welding technology for improving product life spans, maintaining the products during useful life and repairing defects found in products to restore useful life.
Another objective of the invention is to reduce material consumption during welding while reducing the processing time and increasing the performance and life of welded products by replacing or eliminating various required technical operation steps required in the prior art in the production, maintenance and repair of welded products, such as grinding and surface shaping steps.
A further objective of the invention is to produce quality welded joints with consistently controlled stress distribution patterns, which may constitute either initially formed structure during manufacture or redistributed structure and stress patterns initiated during service life.
It is an objective of this invention to introduce life extension methods applicable to welded structural members to substantially increase useful lives and working strengths of the welded products in a manner not heretofore feasible.
It is a further object of the invention to develop scientific methods of treating bodies of metal, plastic and composite materials in a scientifically reproducible manner based upon detected dynamic internal body conditions exhibited during treatment procedures.
Reduction and redistribution of internal stress patterns in work product bodies during manufacture, maintenance and repair, as exemplified for example by treatments of metallic work bodies in the vicinity of weld seams, are achievements of this invention serving to improve service life and load carrying capacity.
The scientific methodology of this invention is achieved in preferred embodiments by improvements in ultrasonic impact technology (UIT) for inducing shock pulses into work products through a transducer (impact tool) in contact with an exterior body surface. More generally, a shock pulse impacts the work body exterior surface by contact with an impact tool transducer to interact in one of the following modes:
(a) To transfer a single impact from a pulse energy source delivered through the transducer into the work body;
(b) To transfer a series of non-periodic impacts into the an work body;
(c) To transfer a periodic train of forced periodic vibrations into the work body; or
(d) To transfer controlled trains of periodic vibrations into the work body as a function of the dynamic conditions of the work body during treatment.
The transducer and its manner of transferring to shock pulse impacts to penetrate the work body structure is a critical tool for generating the necessary amount of internal shock wave energy in the work body, typically steel, to achieve variations of internal body structure, such as restructuring residual stress patterns, temporarily plasticizing the body structure and leaving an improved permanent residual historical change of internal work body structure. This transducer must effectively convert pulse energy from a power source into internal shock waves in the work product body being treated.
One objective of this invention is to introduce pulse wave energy into load bearing work body""s interior structure in such magnitude as to improve the grain structure and the residual stress patterns for producing longer wear and increased load bearing capacity. This requires efficient interchange of energy from an impact pulse source to the interior work body structure. It is selectively desirable to either deform and to avoid deformation of the impact exterior body surface. This can be achieved by different transducer structure.
Typically a surface contact member such as an indenter tool peen or needle is mechanically driven into the surface with a peening function from a mechanically movable transducer body responsive to the available pulse energy derived from a power source in one of the above described modes.
Three basic impact methods can be employed for moving the indenter tool into the work body surface, namely:
(a) one sided contact between one or a set of needle indenters and the treated product surface to drive the needle away from contact with an output working butt of a transducer into the surface being treated for an impulse stroke to spring back to the working butt ready for another stroke;
(b) one sided contact between the needle(s) and the treated product surface awaiting the output impact from the working butt of the transducer;
(c) double sided contact of the needle (s) with both the transducer working butt and the treated product surface; and
(d) Any of the above with a waveguide structure inserted between the transducer working butt inclined toward a treatment position on the product surface to direct the impact energy at an angle or to reach limited access working surfaces.
By selection of these tool combinations significantly increased efficiency of energy impulse transfer into the working body may be achieved.
In any of these impact interactions between a transducer, an indenter and a treated surface, pulsed forces initiated by the UIT process leads to the following factors on the treated body when stroking impulses of appropriate stroke magnitudes and energy content are used:
(a) plastic deformation on the treated work body surface and its internal body volume, typically with penetrations up to 3 mm in steel;
(b) residual compression stresses created equal to or higher than the maximum yield of the treated material in the plastic deformation zone;
(c) residual compression stresses historically stored in the area of elastic deformation, typically up to 5 mm depth in steel;
(d) pulsed compression stresses induced, typically at a depth up to 5 mm from the treated surface;
(e) periodic waves of ultrasonic dynamic stresses induced typically to depths of 12 mm under treated surfaces.
All five effects are initiated by the plastic deformation step (a)
The effects of these UIT factors on the treated body include plastic deformations, residual compression stresses in plastic deformation zones and residual compression stresses in elastic deformation zones leading to redistribution of residual (primarily tensile) stresses, together with reduction of external dynamic and primary tensile stresses.
Both compression stress pulses and dynamic stress waves induced by UIT lead to relaxation of residual internal stresses and external dynamic stresses, sometimes resulting in internal temporary plasticity.
Plastic deformations on and under treated exterior work product surfaces, in combination with redistribution and relaxation of stresses leads to increases in resistance to deformation of body material, aging, fatigue and reductions in various structural defects through the life of a body.
Improved internal work body structure is thus achieved by controlled periodic pulsed energy impact treatment of external work body surface zones, usually nondestructively, to induce internal compression waves resulting in modified internal body material structure, and plastic deformation. One useful embodiment addresses the internal body structure of a welded work product in the vicinity of the weld seam. By introduction of impulse impacts, such as ultrasonic waves of appropriate magnitude and frequency for the work product material being processed, residual stresses are relaxed in depths typically up to 12 mm (for steel). In some instances, depending on UIT parameters, the desirable xe2x80x9cwhite layerxe2x80x9d effect is created.
White layers are formed on the treated body surface and in a narrow under-surface layer, typically one micron thick by interaction of several factors, including:
(a) rapid heating to appropriate temperatures, such as close to annealing temperatures for steel, at the point of ultrasonic impact, as generated by the high repetitive frequency of the impact pulses;
(b) high intensity ultrasonic impact rate inducing plastic deformation with corresponding formation of residual stresses, typically greater than two times maximum yield; and
(c) rapid thermal dissipation from the point of ultrasonic impact at a rate comparable in steel to the cooling rate of steel after annealing.
These factors are achieved by regulation of the frequency and energy of impact and the magnitude of contact stress.
xe2x80x9cWhite layersxe2x80x9d are characterized by substantial absence of evident grain structure in the vicinity of weld seams. It is well known that this amorphous crystalline structure of the xe2x80x9cwhite layersxe2x80x9d forms new grain borders providing fewer internal bubbles or vacancies of material, and dislocations of grain boundaries. The grain boundaries are moved to the borders of the white layer zones. White layers are characterized by higher fatigue resistance and corrosion resistance. The load carrying ability of the white layer structure with redistributed residual stresses thus create higher load bearing strength.
Applied pulsed energy of a random or periodic nature to an external work product surface thus creates compression waves within the work body being treated to dynamically induce a plasticized zone inside the body. Such compression waves achieve a pattern with a gradient stress magnitude pattern extending from the maximum magnitude surface zone receiving the pulsed energy and tapers to reduced magnitudes reaching to an internal unstressed base metal site within the product body. By withdrawal of the pulsed energy, rapid thermal dissipation within the body structure in an annealing manner reorganizes residual stress patterns which reduce overall product load bearing capabilities and introduce zones susceptible to failure and fatigue. For optimum effectiveness the impact treatment is preferably ultrasonically induced under the controlled conditions hereinafter described.
This treatment procedure replaces several technical operation steps required in the prior art initial production stage that simplifies and lowers costs of the treatment of work products, including welded bodies, while improving strength and life expectancy. In welded products the amount of required metal consumption is reduced significantly. Application of pulsed impact energy replaces formerly required such technological operations as: thermal treatment steps including annealing (generally requiring a furnace); overlaying auxiliary welds such as filler welds to increase the loading capacity of a weld joint; mechanical steps of chamfering weld seams to relax internal stresses; peening by hammer peening, needle peening, shot peening, and shot blasting; TIG dressing; abrasive treatment; demagnetizing; attachment of bracing plates; and the like.
Novel process steps and improved welded structures afforded by this invention for treating welded products are illustrated in one preferred embodiment related to the particular technological operation of repair of a crack in a welded body.
In general, this invention corrects prior art deficiencies by reworking the internal micro structure of work product materials in various phases of production, maintenance and repair to relax and redistribute residual structural stress patterns caused by welding in the vicinity of weld seams. Critical stress patterns or concentrations that reduce life and load bearing capabilities of the product are thus eliminated or minimized. By the application of the ultrasonic impact technology afforded by this invention, several predecessor prior art technical operations are eliminated, thus serving to improve the load bearing capabilities of the welded product more simply. Reorganization of residual stress patterns that lead to fatigue, stress corrosion and catastrophic failure improves the work product performance.
This invention provides novel methods not heretofore available in the art to prevent and repair micro structure damage usually encountered during prior art fabrication of new structures. It provides maintenance routines for increasing expected life and renovating fatigue and aging stress patterns. Also, these methods are employed in the repair of visible loading and aging defects encountered in service.
Further novel repair methods are introduced without the high processing costs to effectively eliminate failures and defects due to stress concentration introduced in prior art during sequentially applied technical operations such as stripping and shaping of surfaces, or additional beading and strain hardening procedures such as bending and thermal treatment, all of which require taking the product out of active service.
By the technology of this invention therefore novel work product structures introduced in manufacturing and maintenance procedures are introduced which produces greater load bearing capacity by reducing internal stress patterns. This leads to reduced fatigue failure and longer working life of work products.
Improved instrumentation, treatment methods, products and systems are introduced which produce and exhibit improved internal work product body structure with fewer voids and better internal stress patterns resulting in fewer defective products, service failures and early fatigue in active service. This technology is exhibited in specialty utility embodiments related to the welding arts and weight bearing structural configurations.
First the novel scientific principles employed by this invention are exemplified in the method of treatment of products to reduce internal structural defects that cause premature failure in service.
Thus, interior compressive mechanical vibrations are generally nondestructively induced into the interior body material with a vibrating instrument located on an external surface zone of the body. Appropriate pulse energy, at magnitudes and repetition rates adapted for the particular work body shape and material is applied for plasticizing and reforming resident normally solid interior body material. Such applied energy relates to product shape and utilities and product materials. Thus required energy and pulse repetition rates vary significantly between different metals such as bronze and steel and between metal and plastic bodies, for example. Compressive pulse energy waves induced inside the body relaxes and redistributes residual crystalline structure and product material character to reduce work product voids or bubbles and rework grain structure and residual stress patterns to reform the solid body material structure. The molten or plastic condition, typically serves to relax residual stresses, remove voids and improve grain structure. This improved structure is retained by rapid cooling of the molten material in place achieved simply by withdrawal of the pulse vibration energy.
Controlled and consistently reproducible scientific interior product body restructuring is achieved by this invention which permits adoption of product control conditions and automated procedures for achieving stated objective results, including creation of substantially grainless white layers and relaxing resident internal stress patterns. With this scientific method improved products with longer life spans bearing heavier loads can be produced, maintained and repaired at significantly reduced costs.
Thus, internal product work body conditions are sensed during dynamic treatment to provide feedback signals for controlling and automating the treatment process. Feedback signals are obtained from electrodes located upon the vibrating transducer for electronically sensing the transducer interface loading conditions during dynamic changes in the product body interior structure thereby providing intelligence for control of the effectiveness of the treating process and automating it.
A work product body is treated by the forced driving pulse vibrations induced by an ultrasonic transducer and indenter impact needle at the body exterior surface. Rapid dynamic changes of the internal body material characteristics occur to mismatch the natural vibration frequency of the transducer and the mechanical vibration frequency of the treated body during the treatment procedure. This not only significantly reduces the efficiency of energy transmitted into the treated body, but also significantly jeopardizes the predictable quality of the work product which is processed accordingly under unknown conditions.
There are encountered dynamic changes in surface conditions, internal deflection modes, residual and dynamic stresses within the material structure, which heretofore have not been ascertainable or controllable.
Accordingly this invention by sensing the ultrasonic tool interface conditions with the work product external surface produces a feedback signal indicative of the dynamic state of the body under treatment. For example plasticized or molten metal behaves differently from cold metal, and residual internal stress patterns produce different product characteristics.
The ultrasonic transducer itself by means of carried electrical contact probes thus simply serves as a feedback detector reproducing transducer striction waveforms during the dynamic treatment of the work product. The resulting feedback electric signal then may be analyzed to determine the internal body structure dynamic state and used thereby to maintain a predictable and repeatable product quality level. For example, the efficiency of the ultrasonic treatment in transferring available pulse energy into the work product interior is critical.
The feedback signal of the transducer under forced vibration in contact with the vibrating body surface then will for example produce a periodic waveform pattern representative of the stroke magnitude and the mechanical vibrating frequency at the surface interface. This signal may be periodically be sampled during pauses in the driving oscillation force to the transducer to produce the mechanical resonance frequency and phase. Thus the transducer driving oscillator may be adjusted to the mechanical resonance frequency to significantly increase the transfer of energy by utilization of the tuned circuit Q amplification factor.
This feedback sensing technology is applicable to controlled treatment of a wide range of product body interior structures, including plastic, metallic, ferromagnetic, welded and weldfree work products, resulting in more consistent quality of treated products. Other feedback control signal characteristics are typically useful to control timing of the molten phase and the cooling phase for creating an annealed steel product of particular advantages for example.
Thus, for example, any load bearing work product structure, where the load is compression, tension, thermal or abrasive, can be treated to produce a modified internal body structure that meets the product objectives of longer life and greater load bearing capacity.
Consider for example a ferromagnetic railway brake shoe product braking surface, which is subjected to both compressive load and significant thermal stresses. The treatment method of this invention on the braking surface can produce a stronger, longer life product by processing the interior body structure to remove voids, stress concentrators and restructure as if by annealing the grain structure to produce a longer life white layer surface in the brake surface contact area.
A welded product structure is particularly adapted for treatment by the methodology of this invention to also produce improved product functionality. The stress concentrations at a weld joint between the weld seam and the interior product body structure may be effectively treated to relax stresses, remove voids and produce favorable residual interior stress patterns when cooled in place after thermal treatment.
Furthermore in creating the initial weld seam in the product manufacturing phase, this invention provides automated manufacturing instrumentation, procedures and systems for controlling welding quality and producing a stronger and longer life product. The technique may also be employed during the service life for maintenance to reduce aging and loading fatigue and for repair of visible failures such as bending or striated and cracked surfaces. Accordingly welding technology represents a preferred vehicle for embodiment of various features and innovations afforded by this invention. The improved products, instrumentation and systems afforded by this invention are thus implicitly interwoven in the following embodiments of the invention.
Thus, in accordance with a preferred embodiment of this invention, an ultrasonic impact technology (UIT) surface impact treatment step creates states of plasticity in the inner body structure by way of applied pulsed compressive stress energy. Thus, residual stresses are relaxed and reworked stress gradient patterns result in more effective distribution of internal stress patterns to significantly strengthen the body for its work function. The resulting effect of UIT treatment generally creates a rearrangement submicrostructure of grains in treated areas, particularly in ferromagnetic metals.
More specifically internal structural defects, such as vacancies and grain dislocations, are exposed and moved to the boundaries of modified grain structures such as encountered at weld seams, for example, resulting in annihilation of gradients of structural strain of 2nd and 3rd types in the vicinity of these boundaries. In addition, strain redistribution and reduction of the 1st type strain are introduced in the treated zones, typically encompassing welded joints. This results in welded products with longer life and higher load bearing capacity. Such UIT treatment steps are useful during welding, fabrication of new structures, maintenance operations, and/or treatments of aging, stress fatigue or catastrophic failure to restore life.
In the technical operation of repair of a crack visible on the external surface of a metallic body, the invention is characterized by the basic method steps of UIT treatment supplemented by the mechanical deformation steps of chamfering sharp edges about the crack, drilling holes at crack end points to prevent further spread, and the welding of bracing structure onto the welded product as a further bypass vehicle for bearing load, relaxing internal residual stress defects and favorably influencing dynamics to prevent crack formation and development.
During practical defect repairs in a welded structure, UIT permits the structure to be maintained without interruption of its use (typically bridge support girders over which normal traffic flows) in all phases of maintenance and repair by such procedures as:
(a) repair of deep cracks and full penetration cracks by re-welding and UIT treatment;
(b) repair of shallower surface cracks and defects of structural metal, such as fatigue corrosion defects, by applying UIT treatment to these defects and adjacent areas;
(c) stopping crack development by drilling holes at ends of cracks and chamfering those holes, adjacent zones and hole surfaces with UIT and;
(d) prevention of crack formation or spread by UIT treatment to redistribute stress concentration patterns.
Each of the above steps of structure maintenance comprises sets of operations which in combination result in high quality and reliability of a welded structure. The individual operational steps in these multi-step processes for welded products which are eliminated typically include:
(i) grinding and sanding of surfaces for preparing for welding, drilling and painting;
(ii) mechanical treatment of weldments in order to eliminate stress concentrators;
(iii) mechanical treatment of weldments to remove irregularities of welding joints;
(iv) relaxation of residual welding stresses;
(v) intermediate cleaning of beads from flux, calx and other impurities;
(vi) demagnetizing of welding pass in multi-pass welding procedure; and
(vii) creation of compression stresses for prevention of structural damages of material under normal loading.
Thus, this invention provides simplified procedures that replace a number of conventional procedures, and which is less dependent upon uncontrollable variables in equipment, worker""s qualifications, etc.
In a typical embodiment of the invention,
(i) a UIT transducer working head is located on the surface of a welded work product in a zone residing at a predetermined distance from an applied electric welding arc, lazer beam or other welding torch method in a region having a temperature considerably cooler than the welding temperature.
(ii) In this relationship, the ultrasonic transducer head is caused mechanically to concurrently track by means of appropriate instrumentation a desired surface pattern related to the path of the welding arc. This accordingly creates along the weld seam, as well as in front and/or beyond the weld seam up to the welding arc zone, an internal compression wave pattern which penetrates the welding zone and/or the welded product body far enough and deep enough to reform residual stress patterns within the product body during the welding step.
Introduction of UIT at the actual time of welding results in moving ultrasonic waves through the welding joint and into a molten welding pool. This optimizes the process of welding joint formation and provides its high quality and uniformity in the final structure. The basis for this process is ultrasonic cavitation of the molten metal and acoustic flow which in turn induces ultrasonic outgassing, grain dissipation during its crystallization and optimization of thermal-mass exchange in the welding pool.
Other objects, features and advantages of the invention will be found throughout the following description and claims.